Currently available optical communication receivers are designed to operate over a relatively limited passband of modulation data rates and are not readily tunable over multiple octaves of input signal frequency. Previous attempts to implement a broadband receiver have suffered from limited performance due to the substantial noise figure penalty that accumulates at data rates lower than the highest data rate at which the receiver is intended to operate.
One attempt to solve this problem involves the use of `post` amplification stage, complex noise filtering circuitry downstream of the input signal amplifier. Unfortunately, such filters not only cannot improve upon the noise figure of the pre-amplifier stage, but they introduce an insertion loss penalty of their own, thereby reducing the overall sensitivity of the receiver. Another proposal has been to make the feedback resistor in the input stage's transimpedance amplifier controllably adjustable, for example, by means of a MOSFET-implemented resistor, whose applied gate voltage sets the effective resistance of the feedback resistor. This approach suffers from the fact that such an FET-implemented resistor has more insertion loss that the ordinary passive resistor it replaces. In addition, it introduces stray capacitance and thereby noise into the feedback path. Moreover, it does not provide multiple octave tunability.